CMOS compatible biofet

The present disclosure relates to an integrated chip having an integrated bio-sensor with a sensing well having one or more sensing well spacers that reduce a size of the sensing well after its formation. In some embodiments, the integrated bio-sensor has a sensing device disposed within a semiconductor substrate. A dielectric structure is disposed onto a first side of the semiconductor substrate. The dielectric structure has an opening with a first width, which is exposed to an ambient environment and that overlies the sensing device. One or more sensing well spacers are arranged on sidewalls of the opening. The one or more sensing well spacers expose a bottom surface of the opening to define a sensing well having a second width that is smaller than the first width.